This invention relates in general to transducer assemblies and, in particular, to improvements in methods and apparatus for megasonic semiconductor wafer cleaning.
Megasonic energy and waves are used to clean and remove particles from the surface of semiconductor wafers during wafer processing into devices and integrated circuits. High frequency acoustic energy is termed megasonic for frequencies in the range of 0.5 MHZ and 2 MHZ or higher. Acoustic energy is termed ultrasonic when frequencies range from 20 KHZ to 0.5 MHZ.
Megasonic cleaning is used at many stages in the fabrication process for removing particles, photoresist, dewaxing and degreasing using different solvents and stripping solutions. It has also been shown that megasonic energy will aid in the removal of particulates (.gtoreq.0.1 micron) that are tightly adhered to the wafer surface. The primary advantages of using megasonic cleaning is that it saves in the cost of chemical cleaners, provides superior cleanliness and simultaneously cleans both sides of the wafers, thereby requiring less handling.
Existing megasonic cleaning systems have several drawbacks. In a typical megasonic transducer, a monoclinic quartz piezoelectric crystal is mounted on a quartz plate. The megasonic energy from the crystal is transmitted through the quartz plate into the cleaning solution. The quartz plate may be exposed to the cleaning solution or may transmit the megasonic energy through the tank floor. A typical tank is made of natural polypropylene that does not readily transmits megasonic waves. The thickness of the quartz plate is critical for maximum transmission of the megasonic energy into the cleaning solution.
The back side of the quartz piezoelectric crystal has a bus bar for receiving electrical energy from a cable. The bus bar and the cable connection are typically left open and uninsulated. It has been observed that over time, corrosive fumes escaping from the cleaning solution in the open tank corrode the bus bar and cable connection. While others have completely encapsulated the quartz piezoelectric element, the encapsulation of the surface of the piezoelectric element that faces the quartz plate requires substantial modifications in the size of the plate so that acoustic energy is properly transmitted to the cleaning solution. See, for example, U.S. Pat. No. 5,355,048.
In a typical megasonic cleaning apparatus, one or more transducers are placed at the bottom of the cleaning tank and are substantially in line with one another. These transducers generate columns of standing wave megasonic energy that extend from the bottom of the tank to the top. Studies have revealed that these standing waves often have dead zones or stagnant zones where the megasonic energy has reduced power. If the wafers or portions of the wafers are disposed in those stagnant zones, those wafers or portions of the wafers will not be cleaned as well as the rest of the wafers. In order to remedy this problem, others have proposed moving the wafers from side to side or rotating the wafers. See, for example, U.S. Pat. Nos. 5,427,662 and 5,520,205. Still others have fashioned hollow cylindrical quartz plates with corresponding cylindrical piezoelectric crystals or have provided solid half cylindrical quartz plates fixed to the bottom of the tank for dispersing the sonic energy. See, for example, U.S. Pat. Nos. 4,869,278 and 4,998,549.
Once the wafers have been cleaned, it is important to provide a thin native oxide layer on the wafers as soon as possible in order to prevent contamination of the wafer during its fabrication. Native oxide readily forms on bare silicon wafer surfaces with or without ozone. However when it is formed slowly or in a uncontrolled manner it will tend to incorporate high levels of SiOx particles or other contaminants. Using high levels of ozone (&gt;7 ppm) helps to form a quick and clean native oxide. Such a native oxide layer can be provided by subjecting the wafers to a bath of ozone-rich water. However, current techniques for ozonating water are inadequate. The ozone quickly leaves the waterbath and so the wafers do not receive the desired native oxide layer.